Group III nitride semiconductors typified by gallium nitride (GaN) are wide-gap semiconductors. For example, the forbidden band gaps of gallium nitride (GaN) and aluminum nitride (AlN) are as large as 3.4 eV and 6.2 eV, respectively, at room temperature. Such nitride semiconductors are characterized by having a large breakdown electric field and a higher electron saturation speed than a compound semiconductor such as gallium arsenide (GaAs), silicon (Si), and the like. Then, field effect transistors (FET) using a GaN-based compound semiconductor material have been actively researched and developed as high frequency electronic devices or high power electronic devices.
A nitride semiconductor material such as GaN forms various mixed crystals with AlN or indium nitride (InN), and is therefore capable of forming a heterojunction similar to a conventional arsenic-based semiconductor material such as GaAs. A heterojunction of a nitride semiconductor, for example, an AlGaN/GaN heterostructure is characterized in that a high-concentration and high-mobility carrier is generated at an interface of the heterojunction due to spontaneous polarization and piezoelectric polarization even when the semiconductor is undoped with an impurity. Thus, when a transistor is prepared using a nitride semiconductor, high speed operation can be performed.
Here, AlGaN represents AlxGa1-xN wherein x satisfies 0<x<1; InGaN represents IryGa1-yN wherein y satisfies 0<y<1; AlInN represents AlzIn1-zN wherein z satisfies 0<z<1; InAlGaN represents InyAlxGa1-x-yN wherein x and y satisfy 0<x<1, 0<y<1 and 0<x+y<1. These notations also apply hereinafter.
As a semiconductor device including a nitride semiconductor, in which a gate electrode is formed in a finger shape, i.e., a nitride semiconductor device, one described in Patent Literature 1 has been previously known.